1. Field of the Invention
The present invention relates generally to transmitting and receiving electrical signals through communication channels, such as a gigabit channel. In particular, the present invention relates to a transmit canceller that removes transmit signals from receive signals in such communication channels.
2. Background and Related Art
A gigabit channel is a communications channel with a total data throughput of one gigabit per second. A gigabit channel typically includes four (4) unshielded twisted pairs (hereinafter “UTP”) of cables (e.g., category 5 cables) to achieve this data rate. IEEE Standard 802.3ab, herein incorporated by reference, specifies the physical layer parameters for a 1000BASE-T channel (e.g., a gigabit channel).
As will be appreciated by those skilled in the art, a UTP becomes a transmission line when transmitting high frequency signals. A trans-mission line can be modeled as a network of inductors, capacitors and resistors, as shown in FIG. 1. With reference to FIG. 1, G is normally zero and R(T) is complex due to skin effect. R(T) can be defined by:R(ω)=kR(1+j)√{square root over (ω,)}  (1)where kR is a function of the conductor diameter, permeability, and conductivity. The characteristic impedance of the line is defined by:
                                          Z            0                    =                                                                      R                  ⁡                                      (                    ω                    )                                                  +                                  jω                  ⁢                                                                          ⁢                  L                                                            G                +                                  jω                  ⁢                                                                          ⁢                  C                                                                    ,                            (        2        )            
and at high frequencies, Z0 becomes approximately √{square root over (L/C)} or approximately 100 ohms in a typical configuration. When properly terminated, a UTP of length d has a transfer function H that is a function of both length (d) and frequency (ω):H(d,ω)=edγ(ω),  (3)whereγω=√{square root over ((R(ω)+jωL)(G+jωC))}{square root over ((R(ω)+jωL)(G+jωC))},  (4)
and substituting Equations 1 and 4 into Equation 3, and simplifying, approximately yields:
                              H          ⁡                      (                          d              ,              ω                        )                          ≈                  exp          ⁢                                    {                              d                ⁡                                  [                                                                                                              k                          R                                                2                                            ⁢                                                                                                    ω                            ⁢                                                                                                                  ⁢                            L                                                    C                                                                                      +                                          j                      ⁡                                              (                                                                              ω                            ⁢                                                          LC                                                                                +                                                                                                                    k                                R                                                            2                                                        ⁢                                                                                                                            ω                                  ⁢                                                                                                                                          ⁢                                  L                                                                C                                                                                                                                    )                                                                              ]                                            }                        .                                              (        5        )            Equation 5 shows that attenuation and delay are a function of the cable length d.
A transmission path for a UTP typically includes a twisted pair of cables that are coupled to transformers at both a near and far end, as shown in FIG. 2. A transceiver at each end of the transmission path transmits and receives via the same twisted pair. A cable typically includes two patch cords totaling less than 10 m, and a main section of 100 m or even longer. The transmitters shown in FIG. 2 are modeled as current sources. The near end current source supplies a current Itx. The near end transmit voltage (e.g., ItxRtx) is detected and measured across resistor Rtx. A receive signal Vrcv (e.g., a signal transmitted from the far-end transceiver) is also detected and measured across resistor Rtx. Hence, Vtx includes both transmit (ItxRtx) and receive (Vrcv) signals. Accordingly, the signal Vrcv (e.g., the signal from Transceiver B) received at Transceiver A can be obtained by taking the difference between the transmit voltage and the measured voltage Vtx, as follows:i. Vrcv=Vtx−ItxRtx.  (6)
Conventional solutions for removing transmit signals from receive signals often employ known transconductor (“Gm”) summing stages or other current based methods. As will be appreciated, these methods often introduce signal distortion into the receive signal. Also, some transconductors have a limited signal dynamic range. Accordingly, conventional methods are often inadequate for applications requiring signal recovery. Additionally, known summing circuits, such as weighted summers using operational amplifiers, have not heretofore been modified to accommodate the intricacies associated with canceling transmit signals or regulating baseline wander (described below). A known weighted summer is discussed in Chapter 2 of “Microelectronic Circuits, Third Edition,” by A. S. Sedra and K. C. Smith, 1991, incorporated herein by reference.
As will be appreciated by those skilled in the art, the receive signal Vrcv typically contains additional components, due to baseline wander, echoes and crosstalk, for example.
Baseline wander is preferably corrected for when transmitting and receiving signals over transmission lines. Removing DC components from a receive signal using transformer coupling can cause baseline wander. As will be appreciated by those skilled in the art, baseline wander represents a deviation from an initial DC potential of a signal.
“Echoes” typically represent a residual transmit signal caused by reflections that appear in the receive signal. Echoes can cause undue interference depending on the size of the reflection.
Capacitive coupling between the channels, as shown in FIG. 3, causes crosstalk. Four channels TX1-TX4 are shown in FIG. 3. The capacitive coupling between TX1 and each of TX2, TX3 and TX4 are modeled by capacitors C1-2, C1-3, C1-4, respectively. The capacitive coupling forms a high-pass filter between channels and therefore crosstalk contains mostly high frequency components. As will be appreciated by those skilled in the art, normally only the near-end crosstalk (NEXT) needs to be considered, since crosstalk is usually small and the transmission line provides further attenuation of the far-end crosstalk (FEXT).
Accordingly, there are many signal-to-noise problems to be solved in the art. Hence, an efficient transmission canceller is needed to remove a transmit signal from a receive signal without introducing excess signal distortion. An electrical circuit is also needed to subtract a transmit signal from a receive signal. There is a further need of an electrical circuit to correct baseline wander.